# which solar thermal system is the best? - Page 5

Posted by Gary on July 29, 2006, 2:35 am

Hi Harry,

"The New Autonomous House", Brenda and Robert Yale

Gary

Harry wrote:

www.BuildItSolar.com
gary@BuildItSolar.com
"Build It Yourself" Solar Projects

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Posted by nicksanspam on July 27, 2006, 7:42 pm

Direct gain, with windows on the living space?

G = 3412/40 = 85 Btu/h-F seems low for a house that's larger than
a phone booth with windows and air leaks.

US R48 walls and an R60 ceiling and 8% of the floorspace as R4 windows and
30 cfm of air leaks would make the conductive heat loss about 55 Btu/h-F,
so an 8' tall by A ft^2 square house might have

55 = A/R60+(8x4sqrt(A)-0.08A)/R48-0.08A/R4, ie
A^2 -3505A + 2468K = 0 and A = 976 ft^2,

with a ceiling conductance of 16 and a window conductance of 20 and a
non-window wall conductance of 19 Btu/h-F.

Brick has about 25 Btu/ft^3-F, so 750 ft^3 would have C = 18.7K Btu/F, with
RC = 18.7K/85 = 220 hours. If the house temp starts at 80 F, it would cool
to 60 after -220ln((60-30)/(80-30)) = 113 hours, ie 4.7 days. Warming bricks
with limited surface may overheat the house on a sunny day.

You probably can't get very close to 100% with direct gain. Your body heat
and indoor electrical energy use will help.

More mass under the ceiling could help, eg 4" thinwall PVC water pipes.
With a foil surface under the pipes and the ceiling, they can be hot
without overheating the room. You can bring down warm air as needed
with a slow ceiling fan controlled by a room temp thermostat with
a night setback and an occupancy sensor.

You might have an insulated wall between the living space and the conservatory
and circulate air between them during the day and stop the air circulation at
night and on cloudy days and let the conservatory stay cool.

Nick

Posted by DJ on July 20, 2006, 2:14 pm

Harry wrote:

It is the fastest way, certainly, if you have access to the materials
and place no value on your own time; not being sarcastic, but those are
the realistic parameters.

Site inspections I do on homes comes up, in our area (E.Ontario, SW
Quebec) about 40% of the energy bill.

Anywhere from 6-10K CAD installed for a three-four person system.

Depends greatly on the water useage, but Natural Resources Canada
quotes their studies being less than 10 year ROI.

True enough, but I am doing regular maintenance on systems that were
installed in the early EIGHTIES, vacuum tube and flat plate both.

Inititially, but then with a solar powered pump, you usually do without
a temperature controller, as the panel serves that purpose.

Not a horrible plan, really. The downside is that homemade stuff tends
to be much larger, so you need the roof space and the esthetic
"flexibility" go go with it ;-).

DJ

Posted by Harry on July 19, 2006, 6:38 pm
I think a lot depends on what part of the world you live in.  If
there's a lot of cloudy weather as with us in the UK the evacuated
tubes are the ones to go for. Also we're a long way to the North, most
of the time our sunshine is pretty weak.
I suppose if you live in California, weak sunshine almost never happens
:-)
So the ones for us here to go for is the evac tubes.  Except on bright
days with the flat plates you only get warm water here and have to
boost the temperature with other means.  It means we can get hot water
for more days per year (even if there's less energy collected in
total).
******************

Gary wrote:

SRCC

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Posted by Gary on July 20, 2006, 3:06 pm
Harry wrote:

Hi Harry,
I don't mean to beat this to death, but here is some more data.

I have simulation that I am using to design my space space heating system.
It consists of 240 sqft of solar collectors, a 500 gallon hot water storage
tank, a house that has an effective thermal mass of 6000 lbs of water and losses
350 BTU/hr-sqft-F.  I run this with TMY2 weather files which provide hour by
hour data (sun, temperatures, ...) for a typical full year of weather for the
location you choose.  I usually use Billings, MT since its close to me, but I
did some runs or Buffalo, NY which has the reputation of having the cloudest
winters in the US.  The simulation works on an hourly basis, and calculates the
collector efficiency for each hour based on the solar radiation and temperatures
for that hour -- that is, it gives full credit to evac tubes efficiency under
cloudy condtions.

I did runs for a Heliodyne Gobi flat plate and two evac tubes (Sunda and
Apricus) -- I got the efficiency curve parameters out of the SRCC test
directory.  The simulation calculates lots of stuff, but perhaps the most
relevant for this discussion is the Solar Fraction, which is the fraction of the
house heat that comes from the solar collectors.

The results are pasted in below, but in essence the Heliodyne and the best of
the two evac tubes (Sunda) are tied for performance.

Brand        Solar Fraction    Solar Fraction
Billings, MT      Buffalo, NY

Heliodyne FP    0.39            0.24
Sunda evac      0.39            0.24

Its odd that the numbers come out so close, but if you look at the month by
month summaires below, there are differences by month -- they just end up
balancing out to no difference for the full heating season.  The other evac tube
did less well -- down about 4 or 5 points on solar fraction.

To me, this again says that its not worth spending a lot of extra money to get
evac tubes -- if you can get them for around the same price thats fine? All in
all, I'm just not seeing any big advantage to the evac tubes -- maybe I'm
missing something??

Heating Season summaries:

Frac is the Faction of heat provided by solar
QsolIn is heat from collectors
QhouLoss is the house heat loss
Qfurnace is the portion of heat supplied by furnace
Efic is the collector efficiency (only meaningfull on hourly output)

Sunda Evac Tube -- Buffalo
--------------------------
Title: Evac, singl Glaz, selective, Sunda Town: Buffalo
House: Loss =      350  Whouse =     6250
Collector: Area =  240  Azimuth = 0  Tilt = 70 Yincept =  0.53 Slope =  -0.30
Storage: Wstor =     5000

TstorHigh    =  175.2    TstorLow    =   85.0
TambientHigh =   75.9    TambientLow =   -6.0

Mon    Qout      Qtrans  Efic   Frac QhouLoss  Thou  QsolIn  Qfurnace
M   1  1558388   1558388  0.00  0.15 10196420     0  1558388  8638036
M   2  1416914   1416913  0.00  0.14 10074830     0  1416913  8657921
M   3  2716092   2604745  0.00  0.34  7639912     0  2604745  5035168
M   4  2805457   2752537  0.00  0.56  4881169     0  2752537  2128633
M  10  2296342   2148765  0.00  0.61  3530987     0  2148765  1350971
M  11  1049019   1056083  0.00  0.17  6254924     0  1056083  5198841
M  12  943257    943257   0.00  0.10  9305793     0   943257  8362537
Y   0  12785470  12480700 0.00  0.24 51884080     0 12480700 39372140

Title: Evac, singl Glaz, selective, Sunda Town: Billings
House: Loss =      350  Whouse =     6250
Collector: Area = 240  Azimuth = 0  Tilt = 70 Yincept =0.53 Slope = -0.30
Storage: Wstor =     5000

Sunda Evac Tube -- Billings
---------------------------
TstorHigh    =  174.3    TstorLow    =   85.0
TambientHigh =   82.9    TambientLow =  -18.0

Mon   Qout      Qtrans  Efic  Frac  QhouLoss  Thou   QsolIn Qfurnace
M   1   2580727   2622920  0.00  0.24 10719580     0  2622920  8096657
M   2   2853203   2853203  0.00  0.32  8977437     0  2853203  6124236
M   3   3808663   3808661  0.00  0.49  7733187     0  3808661  3924526
M   4   3188532   2831238  0.00  0.63  4466788     0  2831238  1635549
M  10   3828584   3237756  0.00  0.92  3508342     0  3237756   239335
M  11   2853841   3115152  0.00  0.37  8511174     0  3115152  5396021
M  12   2494588   2498574  0.00  0.24 10504120     0  2498574  8005543
Y   0   21608160  20967540  0.00  0.39 54420760     0 20967540 33421910

Heliodyne FP -- Billings
------------------------
Title: FP, singl Glaz, selective, Heli Town: Billings
House: Loss =      350  Whouse =     6250
Collector: Area =      240  Azimuth =      0  Tilt =     70 Yincept =   0.74
Slope =  -0.81
Storage: Wstor =     5000

TstorHigh    =  174.8    TstorLow    =   85.0
TambientHigh =   82.9    TambientLow =  -18.0

Mon       Qout      Qtrans   Efic  Frac QhouLoss  Thou  QsolIn   Qfurnace
M   1     2633980   2706425  0.00  0.25 10719580     0  2706425  8013152
M   2     3006433   3006432  0.00  0.33  8977437     0  3006432  5971006
M   3     3934056   3934055  0.00  0.51  7733187     0  3934055  3799132
M   4     3183692   2829673  0.00  0.63  4466788     0  2829673  1637113
M  10     3836067   3195664  0.00  0.91  3508342     0  3195664   281427
M  11     2988173   3171250  0.00  0.37  8511174     0  3171250  5339925
M  12     2542469   2565874  0.00  0.24 10504120     0  2565874  7938243
Y   0     22124880  21409400 0.00  0.39 54420760     0 21409400 32980050

Heliodyne FP --- Buffalo
------------------------
Title: FP, singl Glaz, selective, Heli Town: Buffalo
House: Loss =      350  Whouse =     6250
Collector: Area =  240  Azimuth =  0  Tilt = 70 Yincept = 0.74 Slope =  -0.81
Storage: Wstor =     5000

TstorHigh    =  170.3    TstorLow    =   85.0
TambientHigh =   75.9    TambientLow =   -6.0

Mon      Qout    Qtrans  Efic  Frac QhouLoss  Thou   QsolIn Qfurnace
M   1     1547330   1547330  0.00  0.15 10196420     0  1547330  8649094
M   2     1307095   1307095  0.00  0.13 10074830     0  1307095  8767739
M   3     2749305   2593723  0.00  0.34  7639912     0  2593723  5046190
M   4     2853962   2787904  0.00  0.57  4881169     0  2787904  2093266
M  10     2347807   2144674  0.00  0.61  3530987     0  2144674  1355063
M  11     1060275   1088610  0.00  0.17  6254924     0  1088610  5166314
M  12     964401    964401   0.00  0.10  9305793     0   964401  8341393
Y   0     12830170  12433740 0.00  0.24 51884080     0 12433740 39419060

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